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Microelectromechanical system (MEMS) machining enables
infrared emitters to be built directly on a silicon chip.

In one of the first real-world applications
of photonic crystals, two companies
partnered to produce a photonic
crystal enhanced (PCE) micro-hotplate
device with applications in areas from
military combat identification to commercial
and biomedical gas sensing
technology. Ion Optics, which manufactures
optical-based MEMS gas detection
sensors and wavelength-tuned infrared
emitters, partnered with IMT, which produced
the device using its MEMS prototyping
and production capabilities.

The device is a silicon micromachined
infrared emitter, comprised of a photonic
crystal modified micro-hotplate
that emits thermally stimulated infrared
radiation in a narrow band. The wavelength
of light emitted is specified by the
structure of the photonic crystal, allowing
the device to be tuned to a specific
waveband of interest.

The key enabling technology in the
device is the development of the photonic
crystal, which consists of an array
of about 500,000 micron-scale holes
etched into a metal-coated dielectric
substrate. The submicron photolithography
capabilities used to make the device
enable tight control of the photonic
crystal hole size, resulting in
tolerances of approximately 100 nanometers
(nm). Due to the tight control of
the size and spacing of the photonic
crystal holes, the device demonstrates
excellent infrared emitter properties.
The wafer-level packaging approach was
chosen to improve performance. Previously,
parts had been packaged serially
by vacuum capping individual devices
after the wafer was complete. With a
proprietary bonding technology and a
getter, the device provides <10mTorr
vacuum for its lifetime.

Other advantages of the wafer-level
packaging were space savings from attainment
of the smallest package possible
for the device (chip-sized), low cost
of electrical testing, and low cost of burnin,
since it is performed more efficiently
at the wafer level. The process also eliminated
the need for underfilling of solder
joints with organic materials, and enhanced
the device performance by using
minimum-length inter-connections. The
MEMS device also operates at elevated
temperatures, despite its reliance on
thin-film metals, and remains stable at
temperatures over 350°C.

The device can perform the functions
of a non-dispersive infrared (NDIR) gas
sensor by combining it with a retroreflecting
optical cell. Traditional infrared
gas sensors exploit the fact that most
gases have unique infrared signatures in
the 2-14 micron wavelength region. Because
each gas has a unique infrared absorption
line, infrared gas sensors provide
conclusive identification and
measurement of the target gas with little
interference from other gases. NDIR
sensor systems achieve this by assembling
many discrete components including
light source, optical cell, optical filters,
and detectors.

The PCE device generates tuned narrowband
infrared radiation at the absorption
wavelength of the target gas.
Using this light, the device can make accurate
optical measurements, eliminating
cross-sensitivity and false alarms.

The first application of the technology
is in supporting infrared combat identification;
specifically avoiding “friendly
fire” casualties. The emitter can provide
a readily recognizable IFF (Identify
Friend or Foe) signal in existing thermal
sights. The tenability of the photonic
crystal technology allows the emitters to
provide efficient illumination in the desired
infrared band without detectable
cross-talk into adjacent thermal bands. In
addition to protecting against friendly
fire, the device enables emergency
pickup, trail marking, landing beacons,
and marking of high-value equipment.

This work was done by Ion Optics
and Innovative Micro Technology. For
more information, visit Ion Optics at
http://info.ims.ca/5292-232; visit IMT at
http://info.ims.ca/5292-233.

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